1. Field of the Invention
This invention generally relates to a laser beam scanning device including method for operation thereof which allows laser scanning in desired directions along a scan track. The laser scanner and the method for operation thereof includes a laser source for emanating a laser beam therefrom, and a laser beam deflector assembly for continuously deflecting the laser beam. This invention further includes a scan lens system for collapsing the laser beam to a line scan in desired directions along a scan track.
2. Description of the Relevant Art
Refined advances in laser technology has led to applications in numerous new industrial and consumer products. The most commonly used laser beam scanners include polygonal mirror scanners, galvanometric scanners, holographic scanners and acoustooptic beam deflectors. The most commonly used beam scanners are employed in various applications, such as, in laser printers, laser bar code readers or the like.
All the above-described devices deflect light or laser beams, and each type of scanner has a preference with respect to a specific application. Modern scanners can be classified into three categories on the basis of application: (a) illuminators, (b) collectors, and (c) an illuminator/collector combination.
In, laser printers, for example, a rotating mirror directs the laser beam to a rotating drum so as to inscribe several hundred dots per inch on a corresponding photosensitive cylindrical surface. In a bar code reader, for example, commonly used in supermarkets, a mirror guides the laser beam so as to scan the bar code label on the product. The reflected light is sensed by a photodiode which transforms an optical signal to an electrical signal for computer use. In a device for measuring dimensions a rotating polygonal mirror, and an optical lens form a parallel light bundle which is used for object measurement by a photodiode sensing of the amount of light which is cut off by the object.
The use of a plane mirror or a rotating polygonal mirror is an essential component in modern scanners for precisely directing the laser beam towards the target scan track.
Illustrated in FIG. 1 is a conventional scanning device having a polygonal mirror 1. The polygonal mirror 1 rotates only in one direction, shown with clockwise rotation by an arrow 3. The conventional device illustrated in FIG. 1 further includes a laser source 5, and a beam expander 8 for expanding the laser beam 10 emanating from the laser source 5. The laser beam 12 exiting from the beam expander 8 is then directed to a face 15 of the rotating polygonal mirror 1. The laser beam 12 exiting from the beam expander 8 is a larger diameter laser beam with the outermost rays along the circumference thereof referenced by the two arrows, labeled 18a and 18b in FIG. 1. The two opposing points 18a, 18b along the circumference of the laser beam 12 is deflected from the face 15 of the polygonal mirror 1 resulting in the laser beam 20 being directed onto a scan lens which focuses the laser beam 20 into a laser scan spot 24 along the scan plane 26.
As further shown FIG. 1, the laser beam 20 can be directed onto the scan lens 22 at different angles from the face 15 of the polygonal mirror 1, thereby entering the scan lens 22 by way of a, for example, laser beam 28 or 30, as shown in dotted lines in FIG. 1.
The conventional device illustrated in FIG. 1 which employs the uni-directional rotating polygonal mirror 1 results in a one way scan; i.e., a scan from point a to point b as shown in FIG. 1. When the laser spot 24 has traversed the scan plane from point a to point b, the laser spot 24 then jumps the scan from point b back to point a, and thus leaving the return trip (i.e., a laser scan along the scan plane 26 from point b to point a) unused.
In an application whereby a return trip scan is desirable or, whereby a scan in the x and y directions on a given scan plane, is required, a single rotating polygonal mirror 1 falls short in fully meeting the desired laser scanning requirements. Therefore, in laser scanning applications requiring scanning other than that a unidirectional straight line sweep, a synchronized system of a set of two mirrors is used.
Galvanometric scanning with two mirrors can also be used for sweeping along the x and y directions. However, the use of two mirrors for galvanometric scanning is relatively slow because the inertia of the mirrors limits the scan rate to about 300-600 hertz (Hz).
A rotating polygonal mirror 1 is the most popular scanning equipment component in numerous modern laser scanning devices. However, the required design specifications for such a device are complex and stringent. The required design specifications can be categorized in three groups; namely, (a) mechanical, (b) optical, and (c) physical. In other words, scanner velocity stability, provision for "roll off", and reduction of scan jitter are some of the design concerns of the conventional laser scanning device which employs the rotating polygonal mirror. For the most part, stability for the device requires a minimum aspect ratio of diameter to thickness of 12 to 1. In high scan rates (where the mirror's peripheral speeds exceed 400 feet per second, tool steel or beryllium are desirable because of the higher strengths of these materials.
The stringent design requirements of mirrors and, in particular, the restricted laser scan which is limited to a one-way sweep are the compelling reasons for an introduction of a scanner that completely eliminates a rotating mirror, and is more versatile in its scanning performance.
In order to design a scanner that will be characterized as having the ability to smoothly inscribe different scan track shapes, a fundamental geometrical shape which can be easily collapsed or transformed to a new and desired scan track contour must be found. The fundamental geometrical shape, suitable for facilitating the desired scan shapes, is a cone, the arbitrary cross-section of which is an inscribed circle. The inscribed circle is collapsed into desired scan contours using a scan lens system. A performance comparison between a single rotating polygonal mirror and the scanner of this invention is illustrated in Examples 2-7 of Table 1.
It is a further object of this invention to provide a laser scanning device and a method for operation thereof which is capable of inscribing different scan track shapes.
It is another more particular object of the present invention to provide a laser scanning device and a method for operation thereof having a laser source for emanating therefrom a laser beam in combination with a laser beam deflector assembly for deflecting the laser beam exiting therefrom. Also, the laser scanning device and the method for operation thereof includes a scan lens system for receiving the deflected laser beam from the deflector assembly, and generally collapses the inscribed circle into the desired configurations along a laser scan track.